1. Field of the Invention
The present invention relates to an alternator such as an inverter and the like, and in particular, to the type, positioning, mounting, fixing, connecting method and the like of a smoothing capacitor used in the alternator.
2. Description of the Related Art
FIG. 12 is a block diagram showing the circuit construction of a conventional alternator which converts a DC power source into a three phase alternating current for driving an AC load such as a three-phase AC motor. Taking the case of an electric vehicle as an example, when the vehicle is first started off or accelerated, a switching power module 1 converts the discharge output of a DC power source 8 (battery) from direct current to a three phase current to drive an AC load 9 (three-phase motor). On the other hand, when the vehicle performs regenerative braking, regenerative power from the AC load 9 (three-phase motor) is converted from three phase current to direct current and returned to the DC power source 8 (battery).
Switching elements 2 such as transistors which convert power from direct current to three phase current, i.e, IGBT (Insulated Gate Bipolar Transistor) and MOSFET (Metal Oxide Semiconductor Field Effect Transistor), free-wheel diodes 3 which convert power from three phase current to direct current, snubber capacitors 4 for suppressing a surge voltage occurring in the switching elements 2 section during switching, and a drive circuit section 5 for driving the switching elements 2 are loaded in the switching power module 1.
Here, the main property demanded of the snubber capacitor 4 is good frequency characteristics. Thus, a film capacitor is generally used as the snubber capacitor 4. On the other hand, a smoothing capacitor 107 suppresses voltage fluctuation of the DC power source 8 during switching and smoothes a voltage jump and the like, and thus must have a sufficiently large capacitance. Therefore, an aluminum electrolytic capacitor which can easily provide a large capacitance is generally used as the smoothing capacitor 107.
Furthermore, a control circuit section 6 outputs a control signal to the drive circuit section 5 in the switching power module 1 to control the switching elements 2. Moreover, since the drive circuit section 5 and control circuit section 6 are generally circuits for driving and controlling the AC load 9, i.e., three phase motor and the like, detailed drawings thereof are omitted.
Moreover, FIG. 13 is a partially sectional side elevation view showing the internal construction of a common conventional alternator. In FIG. 13, a switching power module 1, smoothing capacitors 107, a snubber capacitor board 20 loaded with snubber capacitors 4 (not shown), and a control circuit board 19 loaded with control circuits 6 (not shown) are housed in a case 23.
Generally, a distributing board 21 such as a copper bus bar or a copper plate and the like is used for connecting the switching power module 1 and the smoothing capacitors 107 which are electrically connected when the distributing board 21 is connected with screws 22. Also, the snubber capacitor board 20 is generally arranged in the vicinity of a positive electrode (P), negative electrode (N) DC input wiring 10p, 10n and a U phase, V phase and W phase AC output wiring 11 of the switching power module 1, and is at the same time fixed and electrically connected with the screws 22.
The switching power module 1 package is constructed from: a resin switching power module case 13 insert molded with the positive electrode (P), negative electrode (N) DC input wiring 10p, 10n, the U phase, V phase and W phase AC output wiring 11 and driving circuit board connection wiring 12; and a switching power module base board 14. Also, an insulating board 15, such as a ceramic board and the like, loaded with switching elements 2 and free-wheel diodes 3, and a driving circuit board 18 (not shown) loaded with the driving circuit section 5 are housed in the switching power module 1 package.
The switching elements 2 and free-wheel diodes 3 are fixed with a bonding member, such as solder and the like, on the switching power module base board 14 via the insulating board 15 which has a conductor pattern. The switching elements 2 and free-wheel diodes 3 are connected with the positive electrode (P), negative electrode (N) DC input wiring 10p, 10n, the U phase, V phase and W phase AC output wiring 11 and the driving circuit board connection wiring 12 by means of a connecting conductor 16 such as wire bonding and the like. Moreover, the driving circuit board 18 and the driving circuit board connection wiring 12 are electrically connected with solder and the like.
A gel filler 17 is filled between the insulating board 15 and the driving circuit board 18, and there are also cases where a resin, such as an epoxy and the like, is further filled thereon. Moreover, this gel filler 17 protects the switching elements 2, free-wheel diodes 3 and connecting conductor 16 so as to prevent the switching elements from being damaged or malfunctioning due to dust and humidity.
The surface of the driving circuit board 18 on the insulating board 15 side is generally beta grounded to obtain an electromagnetic sealed effect so that the driving circuit section 5 does not malfunction due to switching noise generated from the switching elements 2 during power conversion.
Furthermore, a cooling member 24 for cooling the switching elements 2 by means of air-cooling, water-cooling, oil-cooling and the like is attached to the case 23, and Joule heat generated from the switching elements 2 is dissipated to cooling member 24 via the insulating board 15 and the switching power module base board 14. Thus, the switching elements are cooled. Moreover, detailed drawings of the mounting position and fixing method of a control circuit board have been omitted.
The smoothing capacitor 107 must have a sufficiently large electrostatic capacity because it smoothes the power of the DC power source to be supplied to the switching elements 2. Accordingly, it generally has a large size. When an aluminum electrolytic capacitor is used as the smoothing capacitor 107, since the internal resistance thereof is high, the internally generated heat of the smoothing capacitor 107 is increased by ripple voltage fluctuation of the direct current which occurs during switching.
In order to suppress this generated heat, the structure of the switching power module 1 must be complicated by cooling the smoothing capacitor 107 with the cooling member 24, or the electrostatic capacity must be increased further. Accordingly, conventional alternators have had a drawback in that the surface area and volume of the smoothing capacitor 107 are large, thus increasing the size of the entire apparatus.
Also, aluminum electrolytic capacitors have drawbacks in that they have a narrow operating temperature range, and a short service life due to electrolyte leakage which accompanies poor sealing.
Furthermore, since the surface area and volume of the smoothing capacitor 107 are large, there is a drawback in that electrical wiring for connecting the switching power module 1 and smoothing capacitor 107 must be long. Hence, the wiring inductance between the switching elements 2 and the smoothing capacitor 107 increases, and because there is a danger that the switching elements 2 will be damaged by a large surge voltage occurring during switching, snubber capacitors 4 must be provided in the vicinity of the positive electrode (P), negative electrode (N) DC input wiring 10p, 10n and a U phase, V phase and W phase AC output wiring 11 of the switching power module 1.
Moreover, in the conventional switching power module 1, the surface of the driving circuit board 18 on the switching elements 2 side is magnetically sealed by means of a beta grounding method and the like so as to prevent malfunctioning of the driving circuit section 5 due to radiant noise generated from the switching elements 2, hence there is a constraint in that components can only be mounted on one side of the driving circuit board 18.